This invention relates to grinding wheels made from a ceramic material and, particularly, to wheels which require no bonding medium to adhere and hold the ceramic materials in the structure.
Conventional grinding wheels comprise abrasive grit particles held in a matrix that may be vitreous, metallic or resinous in nature. The function of the matrix material is to give the structure physical integrity and strength so that when contacted with a workpiece, the abrasive grits are held tightly enough to ensure that the workpiece is abraded before the grain is worn down or detached from the wheel. In addition, it is desirable that the wheel have "structure", that is to say, it should have a degree of porosity that is determined by the intended use. This porosity helps to dissipate the heat generated by the grinding action and thus reduce burning of the surface of the workpiece. It also makes easier the removal of swarf generated by the cutting action without clogging of the grinding area.
Conventional grinding wheels are produced using two separate firing cycles. The first cycle fires the abrasive grits to their final crystalline form. The second is used to develop the bond which holds the grits together and to form the wheel. By controlling the amount of bond and abrasive material, it is possible to tailor the wheel to the intended use. Thus wheels for low wheel/work conformity, or high-pressure grinding such as external precision grinding or ball grinding, might use a wheel with a fairly low porosity; for example from 5 to 15% pore volume in the structure, while those where burning is a real problem might use a more open structure with a pore volume of 40 to about 65% or even higher. Grinding wheels do not, as a rule, have porosities greater than this because they need to have a certain structural strength to stand up to the strains of a grinding operation. With somewhat higher porosities, say around 80% and higher, structures typical of filters and catalyst support materials are obtained and these are quite different from those suitable for grinding wheels. Ceramic materials such as alpha alumina are very hard and abrasion resistant. Particles of this material can be induced to sinter together in the absence of a bond material to form a coherent structure. If the structure is only partially sintered such that it contains porosity levels suitable for a grinding tool, the mechanical integrity of the structure is inadequate to withstand the forces of grinding. In addition, the high sintering temperature required for an alpha alumina structure leads to growth of the alpha alumina crystals to such an extent that the grinding performance of a structure comprised of such crystals, which is related to the crystal size, is significantly reduced. If high pressure is used to limit the grain growth at the sintering temperature, the products obtained lack the level of porosity required for a grinding tool.
An alternative approach is to use sol-gel technology to generate microcrystalline (generally sub-micron sized crystal structures), alpha alumina in situ in a mass having the form of the desired grinding tool such as a wheel. Coating a sol-gel derived grinding wheel, or for that matter, casting a sol, gel, slurry or slip, is unsatisfactory because of the significant amount of water which must be removed. The elimination of water is impeded by the thickness of the wheel through which the water must pass and the product may, in fact, be formed with voids or may even be crumbled by the internal forces generated upon drying. The problems are exacerbated if a sol-gel of an alumina precursor is used since this adds chemically bonded water that must be removed before conversion to the final fired alpha alumina form can be achieved. Thus, wheels of the thickness typically needed for grinding applications are difficult to obtain using this technique. Furthermore, the casting process inherently limits the range of porosity that can be attained and the products tend to have low porosity and to lack the internal strength to stand up to heavy grinding pressures.
Nevertheless, there is great attraction in the development of a one-step approach utilizing a single firing step to make grinding wheels and there is little doubt that a casting process for producing grinding wheels without the above disadvantages would have great significance. Such a process would, however, need to be controllable in terms of the porosity of the product obtained, and yet not require the use of a matrix bonding medium.
The present invention provides such a process and results in an abrasive wheel with a number of unique characteristics. The wheels produced by the process of the invention provide the benefits of controlled porosity in the context of a process that is essentially a single step and, consequently, very simple in operation.